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A transaction starting with begin transaction, ending with end transaction, in whose execution just one of the two commands commit work or rollback work is executed, and without data management operations processed after the execution of the commit work or rollback work

The file system is responsible for knowing the structure of the secondary memory in directories and the current situation of secondary memory use. It must identify which blocks are free and which are allocated to files

DBMSs use the file system for the following functions:

The creation (create) and removal (delete) of a file

The opening (open) and closing (close) of a file.

read(fileid,block,buffer) for the direct access to a block of a file which is transcribed to the buffer page.

read_seq(fileid,f-block,count,f-buffer) for sequential access to a fixed number (count) of blocks of a file

During the first phase, the dumpis accessed and the damaged parts are selectively copied from the database. The most recent dumprecord in the log is then accessed

The log is traced forward. The actions on the database and the commit or abort actions are applied as appropriate to the damaged parts of the database. The situation preceding the failure is thus restored

Historically, ordered sequential structures were used on sequential devices (tapes) by batch processes. Data were located into the main file, modifications were collected in differential files, and the files were periodically merged. This has fallen out of use

The main problems: insertions or updates which increase the physical space - they require reordering of the tuples already present

Options to avoid global reorderings:

Leaving a certain number of slots free at the time of first loading. This is followed by ‘local reordering’ operations

Integrating the sequentially ordered files with an overflow file, where new tuples are inserted into blocks linked to form an overflow chain

Each node coincides with a page or block at the file system and buffer manager levels. In general, each node has a large number of descendants (fan out), and therefore the majority of pages are leaf nodes

In a balanced tree, the lengths of the paths from the root node to the leaf nodes are all equal. In this case, the access times to the information contained in the tree are almost constant(and optimal)

Insertions and deletions done by using the search technique up to a leaf page

Insertions are easy when there are free slots in the page

When the page has no available space, a split operation is necessary, allocating two leaf nodes in place of one. A split causes an increment in the number of pointers on the next (higher) level in the tree and may cause further split

A deletion can always be carried out in situ

When a key present in other nodes of the tree is deleted, it is better to recover the successive key and substitute for it

When the deletion leaves two adjacent pages underused, they are concentrated into a single page by a merge operation. A merge causes a decrement in the number of pointers on the next (higher) level in the tree and may cause further merge